Carbon blacks are produced in carbon black reactors in large amounts for a wide variety of different industrial applications. Carbon black reactors generally consist of a combustion chamber, a mixing chamber and a reaction chamber which are arranged along the reactor axis, communicate with each other and form a flow path for the reaction media from the combustion chamber via the mixing chamber to the reaction chamber. In the combustion chamber, a fuel, usually gas or oil, is burned in order to generate a high temperature with the aid of a burner under the addition of pre-heated combustion air. A usually liquid, carbon-containing raw carbon black material, e.g., a carbon black oil is sprayed into the hot combustion gases, at which time a part of the raw carbon black material burns and the rest is converted by thermal splitting into carbon black and residual gas. E.g., hydrocarbons of a highly aromatic composition such as coal tar oils, ethylene cracker residues and other petroleum products serve as raw carbon black material.
The raw carbon black material or hydrocarbon feedstock is usually sprayed or injected into a mixing chamber designed as a narrow area in order to achieve an intensive mixing of the raw carbon black material with the hot combustion gases by means of the great turbulence of the combustion gases prevailing there. This mixture then enters into the carbon black reaction chamber, which usually has a cross-section which is widened out in comparison to the narrow area. The actual carbon black formation process takes place in this reaction chamber from nucleation with subsequent growth of the carbon black nuclei and is stopped downstream by spraying in water.
The physical and chemical processes which occur during the carbon black formation are very complex. The heat of the combustion gases is transferred very rapidly to the atomized droplets of the raw carbon black material and results in a more or less complete evaporation of the droplets. A part of the evaporated raw carbon black material is burned in the excess combustion air. Under these conditions the molecules of the raw carbon black material are dehydrogenated and form carbon black nuclei. The nucleation is essentially limited to a limited spatial range, the nucleation zone, within the reaction chamber directly behind the mixing chamber. In the downstream area of the reaction chamber the carbon black nuclei grow to ball-shaped and needle-shaped primary particles. The primary particles, for their part, congregate under the reactive conditions in the reaction chamber to larger aggregates which are firmly connected to each other. The manner of this congregation is commonly designated as the structure of the carbon black.
Various analytic test methods are utilized in order to characterize the carbon blacks. The most important test methods for the applications are the measuring of the iodine adsorption according to ASTM D 1510 as a measure for the specific surface of non-oxidized carbon blacks, the determination of the CTAB surface according to ASTM D 3765 as well as the DBP absorption according to ASTM D 2414 for measuring carbon black structure. The aggregate dimensions of the carbon blacks are determined according to ASTM D 3849 by image analysis of electron microscope pictures. Further important characteristic quantities of the carbon blacks are the amount of volatile (ASTM D 1620) and the amount of extractable components (DIN 53553) as well as the pH (ASTM D 1512).
The characteristic quantities cited determine the properties of use of the materials modified with these carbon blacks, e.g., the properties of use of rubber or of paints. The different requirements of use can be met only by coordinating the carbon black properties with the particular case of use. Accordingly, modern carbon black reactors must be capable of producing different carbon black qualities by suitably changing their operating parameters.
The significant influencing variables for carbon black formation are the excess of air and of oxygen in the combustion gases, the temperature of the combustion gases and the reaction time or dwell time from the mixing in of the raw carbon black material into the combustion gases until the stopping of the reaction by quenching with water, which is sprayed into the downstream area of the reaction chamber by means of a quenching nozzle. The temperature of the combustion gases is usually adjusted to a value between 1200.degree. and 1900.degree. C. The higher the temperature is, the smaller the formed carbon black aggregates become. The dwell time also influences the distribution of the aggregate size. It can be adjusted in known carbon black reactors by means of flow rate and positioning of the quenching nozzle between 1 ms and 1 s.